Direct sequence code division multiple access (DS-CDMA) communication systems are for use in cellular telephone systems with traffic channels located at 800 MHz and in the personal communication system (PCS) frequency band at 1800 MHz. In a DS-CDMA system, all base stations in all cells may use the same radio frequency for communication. One known DS-CDMA system is defined in Telecommunications Industry Association/Electronic Industry Association (TIA/EIA) Interim Standard IS-95, “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (IS-95).
In addition to traffic channels, each base station broadcasts a pilot channel, a synchronization channel, and a paging channel. The pilot channel or pilot signal is a pseudorandom noise or PN code. The pilot channel is commonly received by all mobile stations within range and is used by the mobile station for identifying the presence of a CDMA system, initial system acquisition, idle mode hand-off, identification of initial and delayed rays of communicating and interfering base stations, and for coherent demodulation of the synchronization, paging, and traffic channels.
The pilot signal transmitted by each base station in the system uses the same PN code but with a different phase offset. The base stations are uniquely identified by using a unique starting phase or starting time for the PN sequences. For example, in IS-95, the sequences are of length 215 chips and are produced at a chip rate of 1.2288 Mega-chips per second and thus repeat every 26⅔ milliseconds. The minimum time separations are 64 chips in length allowing a total of 512 different PN code phase assignments for the base stations.
At the mobile station, the received RF signals include pilot, synchronization, paging, and traffic channels from all nearby base stations. The mobile station must search for pilot signals with the strongest pilot channel. If a mobile station is not on the strongest pilot channel, the mobile station is a candidate for handoff.
Idle hand-off is the process of attaching to and listening to the paging channel of the base station with the strongest pilot as identified by the pilot search. When the mobile station receives a page or accesses the system to place a call, it is important that the mobile station is listening to the page from or tries to access the base station associated with the strongest received pilot. This requires a fast pilot phase searching element, particularly when the mobile station is in motion. The searching element needs to be nimble, that is, able to look across the entire phase space as well as looking only at specific PN offsets. Unfortunately, the long times of the prior art searching mechanism adversely affects the soft handoff performance of the mobile station.
New requirements for mobile stations will require Mobile Assisted Hard Handoff, or MAHHO, capabilities. In current IS-95B/IS-2000, MAHHO functionality requires the mobile station to change the frequency of the radio link as it is handed off from one base station to another. Due to the full duplex nature of the CDMA air interface, this requires breaking the radio link, going to another frequency, looking for pilot signals, measuring the target channel forward link RF quality, returning to the original frequency and reacquiring the pilot to reestablish the link. Depending on the number of PNs and/or search window size, this procedure could take several frames (typical six or seven frames but could be as long as fifteen frame) to acquire a pilot signal, which is unsuitable for MAHHO purposes.
Moreover, in MAHHO, the mobile would determine when to leave the current channel to perform candidate search on target channel. As a result, the infrastructure may not know the timing for when the mobile leaves the source channel. Therefore, after the mobile leaves the source channel, the infrastructure may think there is bad reverse link, and would raise the reverse link power control loop set point unnecessarily. When the mobile returns to the serving channel, it needs to reacquire the forward link before turning on reverse link. Since there is no forward power control during MAHHO period, the base station may not be transmitting with the appropriate power, and since the reverse link is not turned on, the mobile unit can not control the forward link power to help the forward link acquisition either. If the forward link is not acquired in time, a drop call will be experienced.
Accordingly there is a need for fast and accurate pilot signal acquisition that will improve mobile station performance, particularly in a DS-CDMA system during hard hand-off.
Skilled artisans will appreciate that common but well-understood elements that are useful or necessary in a commercially feasible embodiment are typically not depicted or described in order to facilitate a less obstructed view of these various embodiments of the present invention.